Measurements of subsurface electrical resistivity have long been used to differentiate rock layers that contain hydrocarbons from other rock units that do not contain hydrocarbons. For example, resistivity well logs have been routinely used for many years to identify producing intervals within existing well bores. However, more recently, it has been found feasible to use surface surveys, which do not require that a well be drilled, in order to detect subsurface hydrocarbon deposits. That is, information collected on the surface is used to determine the subsurface resistivity distribution in the subsurface—in a non-invasive fashion—beneath a line of receivers, the resistivity being indicative of the presence or absence of such deposits. The ability to detect hydrocarbon deposits from the earth's surface, through electromagnetic methods, is of most interest for purposes of the instant disclosure.
Generally speaking, the subsurface has relatively few large contrast resistivity boundaries. However, rocks units that contain hydrocarbons tend to show a large resistivity contrast in comparison with the rocks that enclose them and, as a consequence, resistivity-based methods have long been viewed as potential direct hydrocarbon indicators. For example, the resistivity of a hydrocarbon bearing formation could be on the order of a few tens of Ohm-m or higher, as compared with the resistivity of the over- and under-lying water-saturated sediments, which have resistivity on the order of 2 Ohm-m or less. Thus, when hydrocarbons are present in the subsurface, resistivity-based methods can potentially be used to detect them when other geophysical methods would not. As a consequence, there is currently a great deal of interest in using electromagnetic (“EM”) methods to locate new sources of trapped hydrocarbons, monitor the hydrocarbon distribution within an existing reservoir (e.g., reservoir monitoring over time), etc.
Although there are a number of EM survey variants, of particular interest for purposes of the instant disclosure are surveys that utilize a “controlled source” (i.e., controlled source electromagnetic measurements, or “CSEM”, hereinafter), as opposed to those that utilize a natural source (“magnetotelluric” surveys). CSEM surveys are conducted in both marine and onshore environments. In a marine environment, the survey is typically conducted with a boat pulling a horizontally disposed electric dipole source, near the sea bottom, above a set of sea bottom receivers. The source is powered by an electrical generator that is situated on the boat. The source will usually be programmed to create a low frequency (or other programmed variation) electromagnetic signal by varying the voltage that is supplied to the electric dipole.
In the case of land surveys, a series of source and receiver electrodes are laid out on the surface of the earth. In some instances, the electrodes are mounted on metal spikes so that they can be pushed into the earth, thereby improving their coupling to the ground. Those of ordinary skill in the art will recognize that local conditions of soil conductivity are crucially important to the quality of this coupling.
The CSEM techniques described below are understood to yield low-resolution, but direct indications, of the presence of hydrocarbons. By contrast, seismic exploration techniques yield higher-resolution images of the subsurface layering, but only indirect indications of the presence of hydrocarbons. A combination of these two types of surveys can be a powerful method of exploration.
Although CSEM surveys come in many forms, such surveys may utilize, as a signal source, a time-varying electrical current that is introduced into the subsurface through electrodes or lines of contact at the surface, making a grounded-dipole source. The electrodes might either be placed in direct contact with the surface of the earth (e.g., in a land survey) or towed behind a boat through the water (e.g., in a marine survey). Such grounded dipoles produce propagating electromagnetic fields which are sensitive to resistive anomalies in the subsurface, and hence may be associated with hydrocarbons. Alternatively, the sources might be current loops, which inductively produce propagating electromagnetic fields; however these are sensitive to conductive anomalies, rather than to resistive anomalies, and hence are more useful in the context of mineral exploration and less so in the petroleum context.
In a conventional CSEM survey, receivers are positioned on the earth's surface—to include receivers positioned on the ocean bottom—which are designed to measure the electric and/or magnetic fields that are induced by the source. These measurements are used to estimate the effective or apparent resistivity of the subsurface beneath the receivers, according to methods well known to those of ordinary skill in the art. Note that, depending on the signal source and survey design, the receivers might be situated at distances from zero to 20 kilometers away from the source. In general, longer offsets are required in order to investigate deeper into the earth.
As the source is activated in the vicinity of the receivers, electromagnetic energy propagates from source to receiver, via a variety of paths, and the variations in the amplitude and phase of these fields are detected and recorded by each of the receivers. Various processing algorithms are then used to estimate the resistivity structure of the subsurface beneath the survey.
In some variations, alternating currents are employed as the signal source, with the polarity of such current being reversed at a selected frequency. Any such surveying, with continuous source operating at one or a few selected frequencies (including harmonics) may be called frequency-domain controlled source electromagnetic surveying (f-CSEM). f-CSEM techniques are described, for example, in Sinha, M. C. Patel, P. D., Unsworth, M. J., Owen, T. R. E., and MacCormack, M. G. R., 1990, An active source electromagnetic sounding system for marine use, Marine Geophysical Research, 12, 29-68, the disclosure of which is incorporated by reference herein as if fully set out at this point.
In the marine context, the most common CSEM acquisition methods all use frequency-domain techniques and, more particularly, they use a continuous source signature that operates at one or a few discrete frequencies; Srnka (U.S. Pat. No. 6,603,313, the disclosure of which is incorporated herein by reference) is a good example, citing several other recent patents and publications sharing this same class of techniques.
Another controlled source technique for electromagnetic surveying may be called transient (or time-domain) controlled source electromagnetic surveying, referred to as “t-CSEM” hereinafter. In t-CSEM, an electrode is used to create an electric current in the same general manner as was discussed previously in connection with f-CSEM, except that the source is impulsive (or of short duration), rather than continuous. For example, the electrode may be charged using a DC (i.e., “direct current”) source, which, after some time, is then shut off, causing an abrupt termination of current. Data is then collected from the receivers during the extended time interval that the current is inactive. Of course, such data display a general decay in the measured voltages as time progresses subsequent to the source shut-off. Note that this is in contrast to f-CSEM methods which collect data from the receivers while the source is active. The time variation of the t-CSEM voltages that are observed after the termination of the source current is used to infer the resistivity structure of the subsurface. T-CSEM techniques are described, for example, in Strack, K.-M., 1992, Exploration with deep transient electromagnetics, Elsevier, 373 pp. (reprinted 1999), the disclosure of which is incorporated herein by reference as if fully set out at this point. For the most part, t-CSEM techniques have traditionally been used in connection with land surveys (see Everett, M. E., Benavides, A., and Pierce, C. J., 2005, An experimental study of the time-domain electromagnetic response of a buried conductive plate, Geophysics 70(1), G1, for an application in the context of exploration for unexploded ordinance).
On land, CSEM acquisition techniques are generally well established (cf., e.g., the textbook by Strack, K. M., Exploration with Deep Transient Electromagnetics, Elsevier, 1999). However, land application of CSEM in the hydrocarbon context has been hampered heretofore by difficulties in survey execution, caused by variable coupling of the sources and receivers to the ground. In the most common application, both sources and receivers are attached to the ground via electrodes, which penetrate the ground as far as is practicable. However, the properties of the ground in the vicinity of the electrodes (e.g., variable soil conditions, water saturation, etc.), especially the source electrodes, affects the coupling to the deeper subsurface, so that, if the coupling is not good, the resulting data is weak and variable, and the subsurface signal is compromised. In particular, the strong and variable mismatch in electrical impedance (between the instruments, the near-by ground, and the deep subsurface) contributes strongly to these acquisition difficulties. (By contrast, in marine CSEM, there is no coupling problem, since sources and receivers are intimately and uniformly coupled to the ocean bottom via seawater.)
Because of this ground-coupling problem, the success of land CSEM techniques has been variable, and they have not historically been proven to be broadly useful for hydrocarbon exploration. However, recently attempts to provide commercial services have been launched; these services have been restricted to the conventional oil basins of temperate and tropical latitudes.
Exploration for hydrocarbons in the Polar regions of the earth is hindered by a myriad of practical difficulties associated with the cold temperatures: men and equipment simply do not function well in such environments. In particular, offshore the floating pack ice makes conventional seismic exploration infeasible, and onshore the surface conditions severely discourage the application of proven methods of exploration. Under these conditions, practitioners have not even attempted to apply CSEM exploration methods, which have proven to be unreliable in less harsh conditions, to these more difficult polar environments.
Heretofore, as is well known in the geophysical prospecting and interpretation arts, there has been a need for a method of using CSEM techniques to obtain a resistivity image of the subsurface that does not suffer from the limitations of the prior art. Accordingly, it should now be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for a method of geophysical prospecting that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.